Electrophotographic photoreceptors having a photosensitive layer consisting mainly of amorphous silicon, so-called amorphous silicon type electrophotographic photoreceptors (hereinafter referred to as a-Si photoreceptors) have recently attracted the attention because the amorphous silicon per se has a possibility of essentially improving durability of conventional electrophotographic photoreceptors and is promising for obtaining a long-life electrophotographic photoreceptor having electrically stable repeatability, high hardness, and thermal stability. Taking these advantages into consideration, various a-Si photoreceptors have been proposed, as described in Japanese Patent Application (OPI) Nos. 78135/79 and 86341/79 (the term "OPI" as used herein means an "unexamined published Japanese patent application").
Inter alia, a-Si photoreceptors having a photosensitive layer of a so-called separated function type have been considered excellent, the photosensitive layer being composed of a charge generating layer producing a charge carrier upon light irradiation and a charge transport layer in which the charge carrier generated in the charge generating layer can be introduced and transferred effectively. Various charge transport layers in such separated function type a-Si photoreceptors are known, as described, for example, in Japanese Patent Application (OPI) Nos. 172650/83 and 219561/83, and they are usually formed by decomposing a mixed gas containing a gaseous silane compound, e.g., silane, disilane, etc., a carbon-, oxygen- or nitrogen-containing gas, and a gas containing a trace amount of a Group III or Group V element, e.g., phosphine, diborane, etc., by glow discharge to provide a layer containing the abovedescribed elements to a thickness of from about 5 to 100 .mu.m.
In the separated function type electrophotographic photoreceptors, characteristics of the charge transport layer, which has the largest thickness in the photosensitive layer, generally contribute to charging properties of the photoreceptors. The electrophotographic photoreceptors in which a charge transport layer is made of a hydrogenated amorphous silicon (hereinafter referred to as a-Si:H) obtained by the above-described glow discharge decomposition of silane compounds show insufficient charging properties as having a charging capacity of about 30 V/.mu.m at the highest. Moreover, they generally have such an extremely high rate of dark decay as about 20%/sec at the lowest, though somewhat varying depending on conditions of use. Therefore, application of these electrophotographic photoreceptors using such an a-Si type charge transport layer has been limited to relatively high-speed systems or has required a specific development system due to insufficient charge potential attained. The charge potential can be heightened by increasing the thickness of the charge transport layer. However, such would result in increase of time required for film formation and, when applied to commonly employed processes for production, cause reduction in yield ascribed to an increase of film defect accompanying the formation of a thick layer, only to provide final products at an extremely high cost.